Presently, civil engineering depends on concrete, masonry stones such as limestone, or naturally cemented sandstones for structural support of small, medium, and large civil engineering construction projects. Yet, in certain locations, such materials are either unavailable or cannot be produced at a reasonable cost. Raw materials, energy, or fuel may not be available in the locale. In addition, natural materials and concrete may not be the best materials for thermal insulation, long-term mechanical stability, and long-term chemical stability. Natural materials can also be difficult to quarry and finish because the same hardness that makes them durable also makes them difficult to work. Natural materials also limit color choice.
A substantial percentage of the sandstone deposits available throughout the world are friable sandstone, which has been considered too fragile to use in construction. The structures of friable sandstone deposits are weak and the friable sandstone can disaggregate during quarrying, handling, or finishing, so that the sandstone becomes sand. This occurs in spite of the fact that the friable sandstone deposits themselves have an inherent structure that makes them a strong structural material.
One example of deposits of friable sandstones that exist throughout the world is quartz arenites. These friable sandstones are typically composed of un-cemented or very poorly cemented sand-sized (2 mm to 0.06 mm) quartz grains with ages ranging from several hundreds of millions of years (Precambrian, Cambrian and Ordovician geological periods), several millions of years (upper Tertiary geological period), or even younger. Quartz arenites are common in the geological record and make up about one-third of all sandstones. One example of an old quartz arenite deposit is the St. Peter Sandstone, an Ordovician period quartz arenite widely distributed throughout the central United States. An example of a relatively young quartz arenite deposit is the Iquitos formation, and equivalent formations, widely distributed in the Amazon basin of South America (Peru, Colombia, Brazil, Ecuador, etc.). These young quartz arenites are essentially cohesionless and are referred to herein as aged sands.
Quartz arenites are sedimentary rocks made up of 90 to 95% sand-sized quartz grains (crystalline silica). They are often very friable, that is, they can be easily disaggregated into the sand-grain components and turn into loose sand with relatively little effort, such as finger pressure or very gentle taps with a hammer. This characteristic is useful for quarrying sand, but a problem for quarrying building materials. The St. Peter Sandstone in north central Illinois is quarried by blasting large faces, which turns the sandstone into loose sand. The sands are then handled as slurries and sorted according to grain size. Demand for friable quartz arenites comes mostly from their use as sand for the glass, petroleum, and sandblasting industries.
Use of friable quartz arenites as sand fails to take advantage of the in situ strength of the material. As the quartz sand grains are deposited during sandstone formation, the overburden pressure is concentrated at the very small areas of contact between individual sand grains as very high normal stresses. In a process called pressure solution, matter from solid silica sand grains dissolves in areas of high normal stress concentration and is moved along the contact boundaries into open pore spaces by diffusion. The dissolved silica either migrates to locations where it precipitates or precipitates on the free grain surfaces near the sites of dissolution, coating the sand grains with quartz overgrowths.
The net result of dissolution is an increase in size of contact areas between the sand grains. The shear resistance of sands (resistance to deformation by externally applied loads) is largely controlled by frictional resistance, and frictional resistance is controlled by the area of contact between the surfaces. Therefore, the sand deposits become stronger with time, although at a slowly decreasing rate.
Other factors besides dissolution that may increase the strength of sand deposits with time are cementation and compaction (reduction of pore space). The combined result of all these three processes over time is called diagenesis, which implies the transformation of sediment (sand) into sedimentary rock (sandstone). In quartz arenites, the largest contribution to their strength comes from pressure solution, and cementation is relatively minimal.
Although friable sandstones have great strength while the constitutive grains remain in their original configuration, the grain structure is easily disturbed and disaggregates readily into sand with relatively little effort U.S. Pat. No. 4,072,019 to L. L. Pearson treats friable St. Peter Sandstone soil in place to prevent the need to shore up excavations and tunnels, but does not suggest use of the stabilized sandstone as structural units in construction.
It would be desirable to have structural blocks made from friable sandstone that would overcome the above disadvantages.